1. Field of the invention
The present invention relates generally to magnetic field sensing devices and specifically to magnetic field sensing devices capable of sensing magnetic fields along two mutually perpendicular axes. Such two axis magnetic field sensors are required in many applications. Electronic compasses, magnetic field probes, and virtual reality are a few examples of applications where two axis magnetic field sensors are useful.
2. Description of the Prior Art
In the past, two axis magnetic field sensors were typically constructed using two single axis magnetic sensors. For example, U.S. Pat. No. 5,247,278 describes a single axis magnetic field sensor including an integral current strap for setting a direction of magnetization. Two of the sensing devices described in U.S. Pat. No. 5,247,278 can be used to form a two-axis magnetic field sensing device. For simplicity, two-axis magnetic field sensing devices will be referred to herein as an x-axis sensor and a y-axis sensor, meaning that the two axes are perpendicular. In the past, the two single axis sensors could be housed in a single package enclosure and oriented so that their sensitive directions were perpendicular to each other. Alternatively, two single-axis individually-packaged die could be mounted on a circuit board with the sensitive axis of each die perpendicular to the other die. There are disadvantages to the use of two single axis die. One disadvantage of this approach is that it requires extra assembly effort either at the package level or at the board level. In addition, it is difficult to locate the two single-axis die so that they are orthogonal to each other. The best control on the orthogonality of the two single-axis parts in high volume manufacture may be on the order of ±1°, which induces the same level error on compass heading.
A magnetoresistive sensor capable of measuring a low magnetic field requires that the magnetic moment be initially aligned in one direction, which usually is perpendicular to the sensitive direction of the sensor. With a uniform external magnetic field to initialize the alignment of magnetic moment, it is almost impossible to have an x-axis and a y-axis sensor on a single chip. In addition, generally a magnetic film used for magnetoresistive sensors will have its own crystal easy axis which is determined by a magnetic field applied during the deposition of the magnetic film. Single axis sensors typically utilize this easy axis and initially align the magnetic moment along it. Single axis magnetoresistive sensors usually have the crystal anisotropic field and the shape anisotropic field in the same direction to guard against magnetic and thermal disturbances and to maintain a stable and low noise sensor output. The stability of a magnetoresistive sensor is determined at least to some extent by how good it maintains a single magnetic domain state after the magnetic field for aligning or setting the magnetization is removed.
An integrated two-axis magnetoresistive sensor must have a sensitive direction in an x-axis and a sensitive direction along a y-axis. This means that at least one of the sensor's directions cannot be aligned with the crystal easy axis direction. Therefore, consideration must be given to how to deal with the crystal easy axis when attempting to construct a two-axis sensor on a single die, and how to initially align the magnetic moment in both an x direction and a y direction.
The advantage of a two-axis sensor on one die is that the orthogonality of the two sensors is controlled by the photolithography method, which has accuracy in the range of about 0.01°.
Thus a need exists for an integrated two-axis magnetoresistive sensor.